Focusing Ring; Focus Mode Switch; Zoom Ring - Canon EF 17-40mm f/4L USM Instruction

2. Setting the Focus
Mode
To shoot in autofocus (AF) mode, set the focus
mode switch to AF.
To use only manual focusing (MF), set the focus
mode switch to MF, and focus by turning the
focusing ring. The focusing ring always works,
regardless of the focus mode.
After autofocusing in ONE SHOT AF mode, focus
manually by pressing the shutter button halfway
and turning the focusing ring. (Full-time manual
focus)
3. Zooming
To zoom, turn the lens' zoom ring.
Be sure to finish zooming before focusing.
Changing the zoom ring after focusing can affect
the focus.
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